What responsibility does a seller have to tell a customer an item has been serviced?

A few months ago I bought an expensive pre-amp from a dealer here on audiogon. This is a long time known dealer with an 100% approval rating.

I went to his home to listen to it and there was one quirk about the design I didn't like about it but I thought I could live with it. I had spent so much time and effort to get it. A pretty long drive. When I brought it home and within 24 hours by the afternoon of the next day I realized I couldn't live with it I called him to return it. 
He didn't want to take it back. On his website he has a 30 day return policy. Unfortunately not being an experienced buyer I did not know there is a hidden tab on audiogon saying no returns. Why is this not stated on the main page?!

I was furious and he said he would take it back if I forced him to but he already had another buyer for it. I felt bad and there was a (hidden) no return policy so I let it go.
I had given to him two excellent pieces, the pre and an integrated, and cash for it so I lost quite a bit of money on the deal. I'm obviously not an experienced savvy customer.
I asked him to just give me the pre back for the piece he sold me and keep the rest but no deal. And he told me the pre I gave him in perfect condition, the one I wanted back, wasn't working so he had to send it in for repair. But I shouldn't worry about it. What?!!! I drove it to him I didn't ship it! And why shouldn't I worry that I gave him a non working component when I told him it was in perfect condition?!

Then about a month later I saw he had it listed for sale. This was about 2 AM and I immediately wrote him asking for it in exchange for the item I bought being that the sale he had fell through. Early the next morning hours later he told me it was sold and he had taken it down.

He did list my item he sold to me on audiogon for a couple of months at his expense but no sale. I finally at an extreme loss as I said was able to use it for credit towards another item being sold by another audiogon dealer. This dealer told me he spoke with the American distributor of the brand who told him the first dealer had sent it to him for repair. This was not told to me. Of course I was outraged. But I hadn't asked. If I had known I would never have bought it.

So this is my question: What obligation does a seller on audiogon have to tell a client an item has been serviced if a dumb customer doesn't ask?

Still burned by the loss of so much money and the jerking around and what seems to be a couple of lies.

I think you need to put this down to haste and not clarifying the terms of the deal.
Caveat emptor. 
There may not be any legal obligations to disclose any previous service, however I do feel a dealer,or even a private seller should have the ethics to let any potential buyers know of any service done to the item. I for one,would be very leary of a dealer that feels the need not to be 100% transparent. 
I'm not understanding what the undisclosed repair has to do with anything.  It sounds like there was no issue with unit operation.  
This post is the definition of “caveat emptor,” or is it simply buyer’s remorse. In any case, the OP is expecting too much from a simple stereo transaction. We all have similar stories, but to answer your question, if the piece functions well then there is no additional guarantee. Things break, but how can the seller predict when that will happen? As far as allowing returns, unless specified in advance, nothing should be assumed to be returnable. Move on!
I think the seller in this instance had an obligation to disclose whether there was something wrong with the unit of which he was aware. Plainly, the seller here knew there was an issue with the piece since he had sent it in for service. Now, that having been said, it appears that the repair was effectuated properly and that there was no issue as to operation of the unit; rather, it seems as if you simply didn't like the sound.  In that circumstance, the failure to disclose the repair was inconsequential. You may just have to chalk it up to the price of an education and move on.